confused on what amp to get?

Ok so im finally upgrading my stock speakers to alpine type r's. I am going to have 5.25 alpine speakers in the front and type r 6x9's in the back. the 5.25 rms at 90 watts and the 6x9 rms at 100 watts. my question is do i need to get a 4 channel amp that RMS's at atleast 100 watts per channel or would a smaller amp that RMS's at 75 watts per channel would be ok. Thank you for your comments. also i was told by a bestbuy salesman that underpowering the speaker can damage it. i dont believe that and just believe hes trying to make a buck. let me know